No Stagnation Region Before the Heliopause at Voyager 1? Inferences From New Voyager 2 Results

TL;DR

This study analyzes anisotropy in cosmic-ray protons from Voyager 2 data to infer solar wind speeds, revealing discrepancies with direct measurements and implications for understanding the heliopause region.

Contribution

Introduces a new technique to derive the radial component of anisotropy from CRS data and compares it with existing measurements to study heliosheath dynamics.

Findings

CRS-derived radial speeds generally agree with Low-Energy Charged Particle data.

Significant differences exist between C-G derived speeds and PLS measurements.

C-G method may underestimate true speeds near the heliopause.

Abstract

We present anisotropy results for anomalous cosmic-ray (ACR) protons in the energy range $\sim$0.5-35 MeV from Cosmic Ray Subsytem (CRS) data collected during calibration roll maneuvers for the magnetometer instrument when Voyager 2 (V2) was in the inner heliosheath. We use a new technique to derive for the first time the radial component of the anisotropy vector from CRS data. We find that the CRS-derived radial solar wind speeds, when converted from the radial components of the anisotropy vectors via the Compton-Getting (C-G) effect, generally agree with those similarly-derived speeds from the Low-Energy Charged Particle experiment using 28-43 keV data. However, they often differ significantly from the radial solar wind speeds measured directly by the Plasma Science (PLS) instrument. There are both periods when the C-G-derived radial solar wind speeds are significantly higher than those measured by PLS and times when they are significantly lower. The differences are not expected nor explained, but it appears that after a few years in the heliosheath the V2 radial solar wind speeds derived from the C-G method underestimate the true speeds as the spacecraft approaches the heliopause. We discuss the implications of this observation for the stagnation region reported along the Voyager 1 trajectory as it approached the heliopause inferred using the C-G method.